The present invention relates to an active type distance measurement apparatus for a camera.
A variety of methods are employed for an automatic focusing (auto-focus) function of a camera. One of these methods is a method wherein a distance to an object is measured using an active type distance measurement apparatus, and a photographing lens is moved in accordance with the measured distance.
FIG. 10 is a view for explaining the principle of the active measuring method. In FIG. 10, reference numeral 1 denotes a distance measurement light projecting lens; 2, a distance measurement light receiving lens; 3, an infrared LED; 4, an object; and 5, a PSD (Position Sensitive Device) as a light-receiving element.
The PSD 5 is a light-receiving element which can provide two current outputs I.sub.1 and I.sub.2 as shown in FIG. 11 when it receives an LED image on its element surface. If a distance from the center of the element surface to an LED image P is given as x, the following relationship is established between the distance x and the currents I.sub.1 and I.sub.2 : EQU x.times.K.multidot.l.sub.N (I.sub.1 /I.sub.2) (1)
K: constant
Assume that an image of an object which is located at an infinity point is formed at the center of the PSD 5, and a photographing lens (not shown) is set to be focused at the infinity point. When the object 4 separated from a camera by a finite distance or less is then subjected to distance measurement, a projected spot image is focused on the PSD 5 separated from the center by the distance x. In this case, the following relationship is established between a distance R to the object 4 and the distance x: ##EQU1## where B is a distance (base length) between the light projecting lens 1 and the light receiving lens 2, and f.sub.2 is a focal length of the light receiving lens 2.
If a position on an optical axis of the photographing lens which can be focused for the object distance R is given as y to have an origin on the optical axis when the photographing lens is focused at the infinity point, y represents a defocusing amount, and R is represented as follows in accordance with the relation (1/R)+{1/(f+y)}=(1/f): ##EQU2## f: focal length of photographing lens
Therefore, from equations (1) to (3), y can be calculated as follows: ##EQU3## More specifically, calculation is made using the current outputs I.sub.1 and I.sub.2 of the PSD 5 to obtain x, and x is substituted into equation (4) to obtain the defocusing amount y.
In recent auto-focus cameras, a variety of attempts are made to extend a possible photographing range, and a strong demand has arisen for proximity photographing. However, in the conventional active method, as the object 4 comes closer, the LED image P moving along the PSD 5 is moved to the left in FIG. 11. As the displacement of the LED image P comes closer to half a length L of the PSD 5 (i.e., L/2), the LED image P begins to fall outside the PSD 5. The LED image P falls outside the PSD 5 finally and precise distance measurement cannot e performed. In this case, the length L of the PSD 5 may be prolonged. However, the length L of the PSD 5 is limited in association with the installation space in a camera and cost.
If light reflected by a near object 4 can be received by the PSD 5 in a predetermined light emission amount of the LED 3, as the object 4 comes closer, the reflection light becomes stronger, and the output from the PSD 5 is increased and saturated over the dynamic range. Thus, accurate distance measurement cannot be performed either. In this case, if the output from the LED 3 is increased in order to improve far-distance measurement performance, the near-distance measurement becomes more difficult.
If the area of an LED chip is simply increased to increase the output from the LED 3, the size of the projected LED image is increased, and a far-distance measurement error may occur. For example, in FIG. 12, assume that an object is located at a position C. If a projected LED image from the LED 3 is radiated on the entire object and is reflected thereby, the center of gravity of the received LED image on the PSD 5 coincides with a point P. However, when only a portion of the projected LED image (hatched portion in FIG. 12) is radiated on the object and is reflected thereby, the center of gravity of the received LED image coincides with a point Q. For this reason, although the object is located at the position C, the positions of the received LED images on the PSD 5 are different, and different distance measurement results are obtained, resulting in a distance measurement error.
As described above, in the conventional distance measurement apparatus, it is difficult to accurately perform both far- and near-distance measurements.